In the days of the Beis Hamikdash, every erev Shabbos twelve challas would be baked in a special shape ("lechem hapanim") and would be placed on the Shulchan in the Heichal on Shabbos morning. These challas would sit on the Shulchan all week long, and would be removed the next Shabbos morning when the newly baked lechem hapanim would replace them. The kohanim had a mitzvah to eat the removed challas each week on Shabbos and on the night of motsaei Shabbos until midnight. Miraculously, the challas tasted delicious as if freshly baked.
The Talmud (Menachos 94b) records two views regarding the required special shape of these challas. One opinion is that they were in the shape of a canoe, pointed at both ends and wider in the middle, and the other view is that they were shaped like a rectangular carton. The Chazon Ish writes that both are correct and that either shape would have been acceptable.
On Shabbos when we recite the beracha of hamotzi at the beginning of each meal, the minhag is that we use challas either in the shape of a canoe or in the shape of a carton. This minhag is based on a passage in the Zohar which states that the challas on Shabbos should remind us of the twelve challas which were placed on the Shulchan every Shabbos. On Yom Tov, when the challas used for hamotzi are not reminiscent of the lechem hapanim, many have the custom to use round challas indicating that these challas are only to reminisce about the miraculous double portion of mohn that fell every erev Yom Tov, and not having any connection with the lechem hapanim.
Based on this passage from the Zohar, many have the practice to have twelve challas on the table when reciting hamotzi over only two. Others have the practice of cutting both challas of the three meals of Shabbos, adding up to a total of twelve half challas to remind us of the twelve lechem hapanim.
On Shabbos when we don't go to work or earn any money it is especially important to remind ourselves of both miracles – the mohn and the lechem hapanim. The mohn was referred to in the Chumash as "lechem min hashomayim – food from heaven." Each family would measure how much mohn was brought home each day, and despite the fact that some collected much more that they need and some collected much less than their family needed, it would miraculously turn out that each family ended up with exactly the amount they needed for all members of the family. We believe in hashgacha pratis, especially in the area of parnassah. The Talmud (Taanis 2A) states that parnassah is one of the areas that Hasem does not delegate to malachim, i.e. to nature. He takes care of each person's parnassah individually. One who believes that his parnassah is determined in heaven will not feel the need to violate Shabbos to earn more, nor to encroach on someone else's parnassah in an inappropriate manner. Our motto is that in heaven records are kept of each individual, and "everyone will receive what is due him, and no one can even touch anything that was intended for another person" (see Yuma 38b).
The miracle of lechen hapanim extends throughout all generations. The food we eat on Shabbos is very tasty even though it was not cooked or baked that day. Just as the mitzvah of the lechem hapanim served as a segulah for parnassah for Klal Yisroel (see Netziv), so too our observance of Shabbos not only will not diminish our parnassah, but will enhance it. The miracles of lechem hapanim and the mohn have followed shomrei Shabbos throughout all generations.
In the Shabbos zemiros we allude to the statement in the Talmud (Beitzah 15b) that if one can't afford to buy special delicacies for Shabbos, he should borrow money from others and can safely rely on Hashem that He will see to it that he will be able to repay the loan. Proper observance of Shabbos will not diminish our parnassah.